1. Field of the Invention
The present invention relates to an improvement of electronic driver circuit used on telecommunication lines and more specifically on telephone signalling lines.
2. Prior Art
In a simple telephone system, phone connection is made to the local telephone company for connection to a calling destination. In a complex system, multiple telephone units are tied into a branch exchange, such being designated as a private branch exchange (PBX). The PBX controls the routing of calls from each telephone unit, normally designated as an extension, to the local telephone company. The PBX allows intra-PBX communication without utilizing the local telephone company. In essence, many telephone units may be utilized, although a limited number of trunk lines exist between the PBX and the telephone company.
A more complex system may employ more than one PBX. A typical situation arises when significant physical distances separate the telephone units. In this instance, it is more efficient to utilize multiple PBXs and have telephone units tie into the nearest PBX. Further, inter-PBX communication may occur without accessing the local telephone company. By providing inter-PBX trunk lines, connections can take place between PBXs without utilizing the local telephone company. Also, PBX's may be connected through signalling units to provide inter-PBX communication when physical distance separating the PBX's is significant.
In a typical inter-PBX system, E and M lines which are well-known in the telephone communication industry, are utilized for signalling between the PBXs. Signalling is necessary for providing the hand-shake to connect the desired extensions. The M-lead provides a voltage or a ground potential for signalling purposes. Prior art electrical circuits have been designed to provide the necessary drive on the M-lead. However, prior art methods have employed cumbersome mechanical relays having relatively short life span and circuits requiring large power dissipation to meet standards required by the Electrical Industry Association (EIA). One form of a prior art circuit requires considerable heat sinking of active devices to dissipate heat because of the large power dissipation of the devices. Use of heat sinks also requires significant physical space.
The present invention provides an electrical circuit to drive the M-lead of a tie trunk without resulting in a large dissipation of power in case of short-circuit faults. The improved driver circuit will not require massive heat sinking of active devices, and therefore will reduce the spatial requirement, yet still meet the EIA standards as regards to M-lead requirements. Further, reliability is improved because of the reduced requirement for maximum heat dissipation. Reduced heat sinking will also result in cost savings.